1. Field of the Invention
This invention relates to the electrolysis of alkali-metal halides, and in particular, it relates to the making of diaphragms intended to replace asbestos in cells for such use. Still more particularly, it relates to a process in which the diaphragms are made of synthetic fiber material rather than asbestos, and the diaphragms exhibit not only satisfactory short-term performance characteristics but also satisfactory service life.
2. Description of the Prior Art
The making of diaphragms for brine-electrolysis cells from asbestos has been widely practiced throughout the world for many decades. Those skilled in the art are familiar with the techniques involved, which include suspending the asbestos in water, brine, or weak cell liquor (aqueous sodium hydroxide) to form a slurry, and then, by drawing a vacuum upon the interior of a cathode screen box and immersed in the slurry, causing the diaphragm to be deposited on the exterior of the cathode screen or mesh, which is then mounted within the cell and put into service. The techniques for making diaphragms of this kind which yield satisfactory performance characteristics (such as a tolerably low cell voltage at a current density sufficiently high, a desirably low chlorate content in the caustic product, a satisfactory current efficiency, and good service life) are well known to those skilled in the art. Now that the brine-electrolysis industry has adopted dimensionally stable anodes, it is necessary for the diaphragm material to give a service life on the order of several hundred days if it is not to become a limiting factor with respect to how long a cell can be operated between renewals. Asbestos meets these requirements, but most of the materials which have heretofore been tried as a replacement for asbestos have failed in some respect. Either the performance characteristics are poor, or they are adequate, but they can be maintained only for a relatively short service life, such as 1 month or less.
Moreover, the desirability of finding a material to replace asbestos has become increasingly apparent in recent years. The mining and handling of asbestos presents a health hazard to the workers dealing with it, and this health hazard can be overcome only by adopting measures to protect the involved personnel which add very considerably to the cost of producing and using the asbestos. Not only from the standpoint of the hazard to the personnel involved, but also from the consideration that the spent asbestos diaphragms must be disposed of (and this creates a pollution problem), the widespread use of asbestos is becoming increasingly regarded as intolerable.
The problems confronting one, however, in arriving at an adequate substitute technology, are formidable.
In the first place, it is not easy to obtain a synthetic substance in a physical form that will approximate the performance of fibers of asbestos. Most of the techniques known hitherto have produced fibers that are relatively too coarse, such as tens or dozens of microns in diameter or similar dimension, where what is needed in order to obtain the permeability desired in the product diaphragm is a fiber much finer, on the order of 1 micron by 4 microns in cross-section or less. The idea that such fibers, made of plastic materials which are "self-bonding" in the sense that these materials will coalesce when heated to a proper temperature and thus afford a diaphragm useful in a chloralkali cell is one which appears in the copending U.S. application Ser. No. 548,684, filed Feb. 10, 1975.
Moreover, the environment in which the synthetic fibrous material must operate is a hostile one. On one side of the diaphragm, there is a hot caustic solution with a temperature of about 90.degree. C. and a pH of 14 or greater. On the other side of the diaphragm is the brine solution, which is also hot but may be, on the contrary, acidic, with a pH of about 2 to 4. During operation, there is a considerable evolution of gas taking place on both sides of the diaphragm, so that the solutions in contact with the diaphragm are also turbulent. It is not simple to find materials of the strength and chemical inertness required to suit them for use in such a hostile environment.
There has been, moreover, another problem. The materials which seem most promising, in terms of strength and chemical inertness, are fluorinated polymers, but they exhibit the concomitant drawback that they are relatively hydrophobic. In contrast, asbestos may be characterized as being hydrophilic. The difficult wettability of the fluorinated polymers is troublesome in that it is difficult to start and maintain a proper flow of liquid through the diaphragm if the diaphragm is difficult to wet. If the diaphragm dewets before (or after) the cell is started, reasonable flow cannot be established through the diaphragm, and the cell is not practically workable. During operation, partial or total dewetting has a similar bad effect. Accordingly, even if a material of suitable chemical resistance and physical strength is found and produced in a sufficiently divided physical form, other problems indicated above must be solved before a technology to replace the existing practice of making diaphragms from asbestos will be available.
In the state of the art, it is obvious from U.S. Pat. No. 3,971,706, that, even working with fibers of polytetrafluoroethylene that are tens or dozens of microns in minimum dimension, and using, if necessary, a slurry-forming technique which requires constant use of a stirrer, it is possible to produce a diaphragm and cause it to operate in a cell which is supplied with brine and which produces chlorine and caustic. The above-mentioned patent teaches one way of dealing with a dewetting problem in a diaphragm-type chlor-alkali cell which has a diaphragm of relatively hydrophobic material, such as polytetrafluoroethylene. Nevertheless, that patent is not to be understood as implying that the diaphragms made with it would at all necessarily give satisfactory service life and good performance characteristics in the commercial production of chlorine and caustic.
The prior art also contains the copending application of Arvind S. Patil and Shyam D. Argade, Ser. No. 548,684, filed Feb. 10, 1975, titled "Thermoplastic Fibers as Separator or Diaphragm in Electrochemical Cells". This application discloses and claims the use of fluorohydrocarbons and other self-bonding thermoplastic materials as diaphragms in electrochemical cells. The application specifically mentions various kinds of fluorine-containing polymer for such purpose, the fibers having a dimension of between 0.05 and 40 microns. It is noteworthy, moreover, that this patent application speaks about "self-bonding" and defines the term in such a way that the diaphragm produced must be heat-treated before being used. Moreover, the patent application does not indicate how long its good performance characteristics could be maintained, and it does not give any basis for selecting, among the various polymers which it mentions, the ones that are suitable for use in accordance with the present invention. The patent application goes on to teach that because of the hydrophobic nature of the thermoplastic fibers, it is necessary to include within the internal structure or matrix of the fibers per se a hydrophilic material to ensure the wetting ability of the fibers, and that the wetting agent used may be of organic or inorganic nature, including the oxyalkylene condensates of ethylene diamine and other polyol surfactants, asbestos, barium titanate, titanium dioxide, or (apparently in solid form) a fluorine-containing commercially available surfactant, such as FLUORAD "FC-126" or "FC-170". It is worth noting that, even with the availability of the above-indicated concepts, which are related to those employed in accordance with the present invention, there was not obtained a technologically satisfactory result, partly because of the failure to select a proper polymer and to put it into a proper physical form before making the diaphragms, but largely because of a failure to grasp the present invention.
Attention is also to be paid to Application Ser. No. 566,911, in the names of Arvind S. Patil and Eugene Y. Weissman, titled "Diaphragms from Discrete Thermoplastic Fibers Requiring No Bonding or Cementing", and filed on Apr. 10, 1975, now U.S. Pat. No. 4,036,729. This patent teaches that even without the heat treatment, various thermoplastic materials which have been put into fibrous form in accordance with a method described in Belgian Pat. No. 795,724, can be made into diaphragms for electrochemical cells. This patent teaches that polychlorotrifluoroethylene is among the materials capable of being so treated, but at the time that that application was filed, polychlorotrifluoroethylene was mentioned only because it was chemically similar to various polymers which had been tried and found in bench-scale tests of relatively short duration to yield satisfactory short-term performance characteristics. Again, it is not to be taken from this application that the problem of providing a satisfactory technology providing a material to replace asbestos for the formation of diaphragms in the electrolysis of brine, had been achieved, as it has been with this invention. It was not, for example, appreciated that there might exist, as there do, certain polymers, such as those based upon polychlorotrifluoroethylene, which exhibit the peculiar property, when placed into an environment of chlorine-cell anolyte or catholyte solution at about 80.degree. to 90.degree. C. for a period on the order of 2 weeks, of developing a pair of surface layers or plies, because of the transformation of certain surface portions of the individual fibers involved into a material of substantially different composition, and that this yields a diaphragm of very substantially increased burst strength and service life.